1. Field of the Invention
This invention relates to a method and an apparatus for manufacturing sheets, such as synthetic resin-made sheet pallets used to mount goods thereon for transport and storage. The invention also relates to a stack of pallets.
2. Prior Art
Various sheets have heretofore been used for various purposes. Such sheets include pallets made of a synthetic resin. The pallets made from a synthetic resin are light in weight and small in size as compared with pallets made of wood, and the former are excellent in loading and storing efficiency. Therefore, the pallets made from sheets of synthetic resin have been widely used.
The prior art pallet made from a sheet of synthetic resin is rectangular in shape. Each corner part of the pallet is circularly arcuate in shape. In the vicinity of one side edge of the surface of the pallet, a ruled line (for example, an elongate recess) is formed. The elongate recess is parallel to the one side edge. The elongate recess is a recessed groove, which is used for bending upwardly the portion of the sheet, defined by the groove and the one side edge, to form a tab portion. A load is laid on the top surface of the pallet. In a push-pull apparatus or the like, the pallet loaded thereon with goods is pulled up onto a platen with the tab portion being held, for transporting the goods.
The pallet of this kind has been manufactured by a punching machine for corrugated cardboards, such as disclosed for instance, in Japanese Patent Appln. Laid-Open Gazette No. 85933/1985, U.S. Pat. No. 4,767,393. The shape of knife-edge of the punching machine has been conformed to the shape of the pallet. The prior art punching machine for the sheet pallet has a lower die the top surface of which is flat, and an upper die. The upper die is provided, on the lower side, with a rectangular frame-shaped knife-edge body which matches the external shape of the pallet, and is also provided with a press piece for forming the elongate recess. The sheet pallets have conventionally been formed by punching a sheet material to obtain sheet pallet blanks and then pressing the blanks by this prior art punching machine.
As stated above, the pallets have usually been formed by the rectangular frame-shaped knife-edge body which matches with the external shape of the pallet. Therefore, the sheet material is naturally a little larger than the pallet to be formed. Accordingly, the yield of products is low and this is disadvantageous from the view point of the cost of products.
Since the knife-edge body has a rectangular frame-like shape and it occupies a large space accordingly, the machine used must be of large size.